1. Field of the Invention
The invention relates to a speech codec employing a code excited linear prediction (CELP) algorithm, and more particularly, to a signal-to-noise ratio (SNR) bit rate scalable speech coding and decoding apparatus and method of speech quality enhancement.
2. Description of the Related Art
Speech codecs employing a code excited linear prediction (CELP) algorithm are currently most popular for use in mobile communication systems. CELP speech codecs are based on linear prediction coding (LPC). Transmission rates and bandwidths of the speech codecs vary according to the type of service to which they are applied.
However, the transmission rates and bandwidths of general speech codecs are set by coding apparatuses, not by decoding apparatuses. Further, when a multicasting, in which a packet is sent from one transmitter to a plurality of receivers over a network, is performed, if the speech codec used on a side of the transmitter has a fixed bit rate, the quality of the packet transmitted to the plurality of receivers, which request a variety of bit rates, may deteriorate.
To solve this problem, speech codecs adopting a bit rate scalable speech coding method has been developed. Such speech codecs configure a bit stream containing base codec information and additional information such that quality of a signal transmitted can be more accurately restored.
Conventional bit rate scalable coding methods are classified into a signal-to-noise (SNR) bit rate scalable method and a bandwidth scalable method.
The SNR bit rate scalable speech coding method codes and decodes a speech signal using hierarchical coding. That is, the SNR bit rate scalable speech coding method codes a speech signal in a base layer and a speech quality enhancement layer, respectively. The base layer transmits only information for restoring the least of the speech quality, and the speech quality enhancement layer transmits additional information for enhancing the speech quality.
However, conventional SNR bit rate scalable coding apparatuses are constructed such that the speech quality enhancement layer is independent of the base layer. Thus, since calculations of energy and a correlation between an impulse response and a target signal (or target vector) necessary for a fixed codebook search are performed respectively in the base layer and the speech quality enhancement layer, a great number of calculations are required to obtain parameters for the fixed codebook search.
Furthermore, since the conventional SNR bit rate scalable coding apparatuses change the structures of existing standard CELP speech codecs to additionally operate the speech quality enhancement layer, the conventional apparatuses are not compatible with the existing standard CELP speech codecs.